1. Field of the Invention
The present invention relates to a method of determining the position of a signal receiving point by processing data included in signals transmitted from artificial satellites in geosynchronous altitude orbits.
2. Description of the Prior Art
Conventional radio navigation methods range from those having oscillation sources on the earth such as Decca, Omega and Loran-C systems to NNSS (Navy Navigation Satellite System) and GPS (Gloval Positioning System) utilizing artificial satellites. Systems utilizing artificial satellites can dispose criteria of position determination in cosmic space. Therefore, it is possible to improve an accuracy of position determination by expanding the scope of position determination and mounting high-grade apparatuses on the satellites. Of these systems, NNSS utilizes the Doppler effect, while GPS utilizes measurement of distance based on time measurement. The latter provides many excellent performances in comparison with the former and will become a leading system in forthcoming navigation satellite systems. In comparison with the present invention, such GPS system will be explained as a typical prior art.
FIG. 1 indicates the arrangement of artificial satellites in GPS. The reference numerals (300), (301), (302), (303) denote NAVSTARs (NAvigation System with Timing and Ranging) for GPS and their positions are indicated as A', B', C' and D'. Each NAVSTAR has a precise atomic clock which generates a highly accurate clock signal and accurately expresses the present time with reference to the beginning of a week through calibration by timing information received from a control station. The control station decides the orbit from tracking data of NAVSTAR, and thus, when a time is once selected, the position of NAVSTAR can be decided. Therefore, if an observer 304 were at NAVSTAR's position, he would detect a current time at NAVSTAR and know the position from the detected time and the orbital information. Next, it will be discussed that an observer 304 take a position P' where he can observe NAVSTARs position. Here, it is assumed that the observer has a timing device which is not so accurate and includes a fixed error relative to a reference time. It is also assumed that the observer performs measurements at a time Tno+.DELTA.t, where Tno indicates a correct current time and .DELTA.t indicates a fixed error inherent to the observer. In this case, assuming that NAVSTARs (300), (301), (302), (303) are observed by the observer at Tn1, Tn2, Tn3 and Tn4, respectively, these times respectively include delays corresponding to the distances between the observer and the satellites.
From the relationship between the observed time and the position of NAVSTAR, the following equations can be established. EQU A'P'=C (Tno+.DELTA.t-Tn1) EQU B'P'=C (Tno+.DELTA.t-Tn2) EQU C'P'=C (Tno+.DELTA.t-Tn3) EQU D'P'=C (Tno+.DELTA.t-Tn4) (1)
where, C denotes the velocity of light.
The equations (1) consist of four kinds of equations including, as unknown numbers, three values representing observer's three-dimensional position and .DELTA.t. Therefore, these equations have solutions and provide the position of the observer, which allows the observer to calibrate his clock.
NAVSTARs move in a circular orbit once in 12 hours, and the altitude of the orbit is 20, 183 km, the orbit inclination angle being 55 degrees. In such an orbit, three satellites are located at equal intervals. Six such orbits have been prepared and therefore 18 satellites in total make the flight along the orbits. These satellites are arranged so that four NAVSTARs can always be observed from any point on the earth.
One control station and four monitor stations are provided in the NAVSTAR system, and when four NAVSTARs exist within a visibility of each station, data can be acquired and necessary commands can be transmitted. Orbital data can be processed with a data processing facility on the basis of the data acquired. For the control of time, the timing device is calibrated by the primary time standard.
In the case of GPS, at least 18 NAVSTARs are required for conducting position determination at every place on the earth. Moreover, the NAVSTAR system requires highly precise time and frequency data in order to generate source data for measurement of range. However, the control station calibrates time and frequency at a period of a week and therefore NAVSTARs require atomic clocks to suppress any fluctuation of time and frequency to within an allowable range, thus resulting in an increase in costs.